The present invention relates to a heat-sensitive transfer medium utilizing a color-forming reaction between a leuco dye and a developer therefor. More particularly, the present invention relates to a heat-sensitive transfer medium which is capable of producing a high density transfer image by a minute energy and further producing uniform density transfer images even when the transfer operation is repeated a large number of times.
As conventional heat-sensitive transfer mediums, there are known one comprising a combination of a transfer sheet having a thermo-sublimating dye layer on a substrate with a receiving sheet receiving a thermo-sublimating due image from the back side of said transfer sheet by thermal printing, and one comprising a combination of a transfer sheet having a transfer layer containing a heat fusible substance and a chromatogenous agent (dye or pigment) on a substrate with a receiving sheet.
However, the former heat-sensitive transfer medium using the thermo-sublimating dye is defective in that the dye image formed on the receiving sheet is inferior in preservability and therefore an overcoat must be applied on the transferred image, while the latter heat sensitive transfer medium using the transfer sheet having the transfer layer formed by dispersing dye or pigment in the heat-fusible substance is defective in that when a large amount of the chromatogenous agent is incorporated in the transfer layer in order to obtain high density images, the transfer efficiency deteriorates, and consequently it is difficult to obtain high density images, and further when a large amount of heat fusible substance is used in the transfer layer in order to raise a heat-sensitivity, said large amount of heat fusible substance transfers to the receiving sheet side, and consequently when stripping the transfer sheet of the receiving sheet, it is not stripped smoothly and the fine-lined up image area becomes indistinct.
On the other hand, there is also known a heat-sensitive transfer medium wherein substances thermally reactive each other to bring out color are respectively carried on separate substrates and the substances are brought into contact for being thermally printed. However, the heat-sensitive transfer medium of this type can be observed defective in that due to its reactive type, a sufficient color-forming reaction can not be carried out in case the transfer layer is merely transferred to the receiving layer at the time when the surfaces of both layers contact, so that the low density images may be produced, and that in case the thermo-printing is carried out for a long period of time under the elevated heating condition, the density of the image on the receiving sheet becomes higher, but on the other hand the color forming reaction progresses also on the transfer sheet thereby to form undesired images.
We proposed that a receiving layer on a receiving sheet should contain porous fillers having a large oil absorption amount to provide an improved heat/sensitive transfer medium having no defects as mentioned above (see Japanese Patent Application No. 57-139347). However, this medium is still unsatisfactory in respect of the stability of images and the density of images although images of a relatively high density can be obtained by repeatedly transferring a large number of times.